Differences between LMSL and MLLW

[David Finlayson]

Keven,

I don't know if this is the only NOS benchmark (tide station) in Willapa
Bay, but it will get you started, scroll down to the bottom for the datum
information.  Check the NOS web site for more benchmark possibilities.

http://co-ops.nos.noaa.gov/benchmarks/9440747.html

As for your question about the difference between MSL (~ NGVD29) and NAVD88,
there is information on the NOS web site explaining the difference between
these datum along with some software that will calculate the difference if
you need to convert from one datum to the other.  Also, many benchmarks are
surveyed into NAVD88 so the work is already done for you.

Keep in mind that MSL (and hence the old NGVD29) was a physical elevation,
measured at the tide station (around 1929), while NAVD88 is a best-fit geoid
that attempts to match MSL for as many places as possible (around 1988), but
which is really a mathematical fiction (the world isn't a smooth spheroid).
Mean sea level is a moving target, that's why they established NAVD88 in the
first place.  Unfortunately, it is tough to survey an imaginary line on the
beach.  In practice engineers survey into a local datum as established by
second and third order benchmarks, it is extremely expensive to place a
first-order benchmark into the ground that ties directly into national
datum.   Thus, most GIS data is actually tied to the local datum and is only
approximately tied to one of the national datum.  (Very expensive GPS can
give you vertical information tied to yet another geoid, WGS84)  Your
application will justify the cost of getting it right.

I would stick to the datum of your most important data.




----- Original Message -----
From: "Harvey Greenberg" <hgreen at u.washington.edu>
To: "UW - GIS Discussion & Support" <uw-gis-l at u.washington.edu>
Sent: Tuesday, September 04, 2001 10:12 AM
Subject: Re: Differences between LMSL and MLLW


> Keven, with at least one of our experts on vacation, I will attempt a
> partial answer.  In Puget Sound (See
>
http://students.washington.edu/~dfinlays/pugetsound/presentation/newsound.ht
m),
> we dealt with the difference between MLLW and MSL.  Those values are
> substantially different, and the difference varies substantially through
the
> Sound.   David Finlayson gathered MLLW values for every tide station, and
> interpolated the values.  This difference "surface" was used to adjust
bathymetric
> data to the same vertical datum as the NAVD29 topographic data.
>
> We also attributed the shoreline with the value of MLLW and used it with
the
> bathymetric points in building a surface.  We were particularly
dissatisfied with
> the interpolation between the lowest terrestrial contour and the shoreline
in
> the creation of the USGS DEMs (not that we are happy with the upland
> interpolations), and looked into replacing part of the USGS DEM with our
> own interpolation.  However, we have not come up with a technique that
works
> well on all natural and altered shorelines.
>
> If you can find the difference between MSL and NAVD29, use it, but I don't
> expect it to be substantial value.  As for geoids, stick to the one that
mathed
> your horizontal datum.
>
>    ~harvey
>
> >     1.  What is the difference between LMSL (local mean sea level) and
> > MLLW in Willapa Bay in meters, on average, within the bay?
> >     2.  I have a paper that explains the evolution and technical issues
> > concerning  controlling the NAVD88 datum, and in it, it explains that
> > there may be local differences between MSL as represented in NAVD88 and
> > LMSL for some locations within North America.  If I find LMSL is
> > different in Willapa Bay from MSL for NAVD88 for that area, does this
> > mean that I should correct the elevation of the DEM for Willapa bay (I
> > have a decimeter sheet given to me by Harvey Greenberg, so maybe he
> > knows), or has this already been done?
> >     3.  If there are updates made to NAVD88, or NAVD29, do I need to
> > grab new geoid info for ArcView and/or Arc/INFO so that corrections
> > don't need to be made?
> >     Thanks
> >
> >     Keven Bennett
>
> --
> hgreen at u.washington.edu  162 Johnson Hall  206-685-7981
> http://duff.geology.washington.edu/
>
>         Harvey Greenberg
>         University of Washington
>         Department of Earth and Space Sciences
>         63 Johnson Hall (Box 351310)
>         Seattle WA 98195-1310
>
>
>
>